Cobalt top layer advanced metallization for interconnects

ABSTRACT

An advanced metal conductor structure is described. An integrated circuit device including a substrate having a patterned dielectric layer. The pattern includes a set of features in the dielectric for a set of metal conductor structures. An adhesion promoting layer is disposed over the set of features in the patterned dielectric. A metal layer fills a first portion of the set of features and is disposed over the adhesion promoting layer. A ruthenium layer is disposed over the metal layer. A cobalt layer is disposed over the ruthenium layer fills a second portion of the set of features. The cobalt layer is formed using a physical vapor deposition process.

BACKGROUND OF THE INVENTION

This disclosure relates to integrated circuit devices, and more specifically, to a method and structure to create advanced metal conductor structures in semiconductor devices.

As the dimensions of modern integrated circuitry in semiconductor chips continues to shrink, conventional lithography is increasingly challenged to make smaller and smaller structures. With the reduced size of the integrated circuit, packaging the circuit features more closely together becomes important as well. By placing features closer to each other, the performance of the overall integrated circuit is improved.

However, by placing the integrated circuit features closer together, many other problems are created. One of these problems is an increase in the resistance-capacitance (RC) delay caused at least in part by the increase in copper resistivity as the dimensions of the features become smaller. The RC delay is the delay in signal speed through the circuit as the result of the resistance and capacitance of the circuit elements.

The present disclosure presents improved interconnects to alleviate this problem.

BRIEF SUMMARY

According to this disclosure, an advanced metal conductor structure is described. An integrated circuit device including a substrate having a patterned dielectric layer. The pattern includes a set of features in the dielectric for a set of metal conductor structures. An adhesion promoting layer is disposed over the set of features in the patterned dielectric. A metal layer fills a first portion of the set of features and is disposed over the adhesion promoting layer. A ruthenium layer is disposed over the metal layer. A cobalt layer is disposed over the ruthenium layer fills a second portion of the set of features. The cobalt layer is formed using a physical vapor deposition process.

The foregoing has outlined some of the more pertinent features of the disclosed subject matter. These features should be construed to be merely illustrative. Many other beneficial results can be attained by applying the disclosed subject matter in a different manner or by modifying the invention as will be described.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings which are not necessarily drawing to scale, and in which:

FIG. 1 is a cross-sectional diagram depicting the dielectric structure after patterning and etching steps have been performed according to a first embodiment of the invention;

FIG. 2 is a cross-sectional diagram depicting the substrate structure after a liner deposition step has been performed according to a first embodiment of the invention;

FIG. 3 is a cross-sectional diagram depicting the structure after a first metal deposition step has been performed according to a first embodiment of the invention;

FIG. 4 is a cross-sectional diagram depicting the structure after a ruthenium (Ru) deposition step has been performed according to a first embodiment of the invention;

FIG. 5 is a cross-sectional diagram depicting the structure after a cobalt (Co) deposition step has been performed according to a first embodiment of the invention;

FIG. 5 is a cross-sectional diagram depicting the structure after a thermal anneal step has been performed according to a first embodiment of the invention;

FIG. 6 is a cross-sectional diagram depicting the structure after a second metal deposition step has been performed according to a first embodiment of the invention;

FIG. 7 is a cross-sectional diagram depicting the structure after a planarization step has been performed according to a first embodiment of the invention;

FIG. 8 is a cross-sectional diagram depicting the structure after a planarization step has been performed according to a second embodiment of the invention;

FIG. 9 is a cross-sectional diagram depicting the structure after a planarization step has been performed according to a third embodiment of the invention; and

FIG. 10 is a cross-sectional diagram depicting the structure after a planarization step has been performed according to a fourth embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

At a high level, the invention provides a method and resulting structure to form interconnects which reduce resistance-capacitance (RC) delays as compared to conventional interconnects. In the invention, a multiple fill metallization is used, where ruthenium and cobalt layers are used as the top layers of an interconnect stack. The ruthenium/cobalt combination is chosen as cobalt has a better lattice match with ruthenium than alternative materials such as titanium or tantalum. This provides a good interface for performing a reflow process. The combination of material provides good metal fill properties in aggressively scaled features, e.g., less than twenty nanometers. With good metal fill properties, the reliability of the interconnect is also improved.

Instead of a complete feature fill in a single deposition step, a partial fill with one metal followed by a second fill with a second metal has benefits for both process control and deposited material quality because of the reduced aspect ratio for each of the fills. The reduced aspect ratio of the remaining structure is another advantage for the second metal fill. In some embodiments of the invention, the first metal fill is copper. Having Ru/Co deposited and passivated on a Cu layer provides an improved electromigration resistance because of a reduced Cu surface migration. Another advantage of choosing Co as the second filling material is that narrow Co features display comparable resistance to Cu features of comparable size about 40 nm or smaller features.

A “substrate” as used herein can comprise any material appropriate for the given purpose (whether now known or developed in the future) and can comprise, for example, Si, SiC, SiGe, SiGeC, Ge alloys, GaAs, InAs, InP, other III-V or II-VI compound semiconductors, or organic semiconductor structures. Insulators can also be used as substrates in embodiments of the invention.

For purposes herein, a “semiconductor” is a material or structure that may include an implanted impurity that allows the material to sometimes be conductive and sometimes be a non-conductive, based on electron and hole carrier concentration. As used herein, “implantation processes” can take any appropriate form (whether now known or developed in the future) and can comprise, for example, ion implantation.

For purposes herein, an “insulator” is a relative term that means a material or structure that allows substantially less (<95%) electrical current to flow than does a “conductor.” The dielectrics (insulators) mentioned herein can, for example, be grown from either a dry oxygen ambient or steam and then patterned. Alternatively, the dielectrics herein may be formed from any of the many candidate high dielectric constant (high-k) materials, including but not limited to hafnium oxide, aluminum oxide, silicon nitride, silicon oxynitride, a gate dielectric stack of SiO2 and Si3N4, and metal oxides like tantalum oxide that have relative dielectric constants above that of SiO2 (above 3.9). The dielectric can be a combination of two or more of these materials. The thickness of dielectrics herein may vary contingent upon the required device performance. The conductors mentioned herein can be formed of any conductive material, such as polycrystalline silicon (polysilicon), amorphous silicon, a combination of amorphous silicon and polysilicon, and polysilicon-germanium, rendered conductive by the presence of a suitable dopant. Alternatively, the conductors herein may be one or more metals, such as tungsten, hafnium, tantalum, molybdenum, titanium, or nickel, or a metal silicide, any alloys of such metals, and may be deposited using physical vapor deposition, chemical vapor deposition, or any other technique known in the art.

When patterning any material herein, the material to be patterned can be grown or deposited in any known manner and a patterning layer (such as an organic photoresist aka “resist”) can be formed over the material. The patterning layer (resist) can be exposed to some form of light radiation (e.g., patterned exposure, laser exposure) provided in a light exposure pattern, and then the resist is developed using a chemical agent. This process changes the characteristic of the portion of the resist that was exposed to the light. Then one portion of the resist can be rinsed off, leaving the other portion of the resist to protect the material to be patterned. A material removal process is then performed (e.g., plasma etching) to remove the unprotected portions of the material to be patterned. The resist is subsequently removed to leave the underlying material patterned according to the light exposure pattern.

For purposes herein, “sidewall structures” are structures that are well-known to those ordinarily skilled in the art and are generally formed by depositing or growing a conformal insulating layer (such as any of the insulators mentioned above) and then performing a directional etching process (anisotropic) that etches material from horizontal surfaces at a greater rate than its removes material from vertical surfaces, thereby leaving insulating material along the vertical sidewalls of structures. This material left on the vertical sidewalls is referred to as a sidewall structure. The sidewall structures can be used as masking structures for further semiconducting processing steps.

Embodiments will be explained below with reference to the accompanying drawings.

FIG. 1 is a cross-sectional diagram depicting the dielectric structure after patterning and etching steps have been performed according to a first embodiment of the invention. Although only a single damascene feature 102 is shown for ease in illustration, the patterned dielectric structure could include a set of vias, a set of trenches, or combination of the same in different embodiments of the invention. An interconnect formed in a via is used to conduct current between the device and conductive line layers, or between conductive line layers. An interconnect formed in a trench is part of a conductive line layer which conducts current parallel to the substrate. As is known, a photoresist or sacrificial mandrel layer can be patterned over a dielectric layer. The subsequent etch will create the dielectric structure depicted in FIG. 1. The dielectric layer 101 is silicon dioxide in preferred embodiments, however, other dielectric materials are used in other embodiments of the invention. Further, the dielectric layer 101 is preferably part of a multilayer structure comprising a plurality of materials.

The single damascene structure 102 shown in FIG. 1 has been etched into the substrate with an aspect ratio (H/D) of height (=H) to width (=D). The example illustration shown in FIG. 1 could be a via or a trench. In some embodiments of the invention the range of aspect ratios is 0.5 to 20 with aspect ratios of 1 to 10 being preferred. However, in the actual device, there may be high aspect ratios (Height/width) which are greater than 20:1. A typical range of heights of the patterned structure (or depths of the patterned structure) H is from 100 nanometers to 10 micrometers and a typical range of widths of an individual feature D is from 5 nanometers to 1 micrometers.

FIG. 2 is a cross-sectional diagram depicting the substrate structure after a liner deposition step has been performed according to a first embodiment of the invention. In preferred embodiments of the invention, a liner material selected from the group of Ta, Ti, W, Ru, Co, their nitrides or a combination of the same is deposited. The liner material is deposited as a barrier layer 103 over the patterned dielectric layer 101 utilizing any conventional deposition process including, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD) or sputtering. The thickness of the layer 103 can vary according to the type of layer being formed and the technique used in forming the same. Typically, the layer 103 has a thickness from 1 nm to 100 nm with a thickness from 2 nm to 20 nm being more typical. The liner material 103 prevents the diffusion of the subsequent metal layer, e.g., a copper layer, into the dielectric 101, acting also as an adhesion promoting layer so that the copper metal layer is bonded to the substrate. Experimental results have shown that direct deposition of Cu on the dielectric produces poor adhesion and causes delamination related reliability problems.

FIG. 3 is a cross-sectional diagram depicting the structure after a first metal deposition step has been performed according to a first embodiment of the invention. In preferred embodiments of the invention, the first metal deposition is a copper deposition step. However, in other embodiments of the invention, the second metal is selected from the group consisting of Al, Ni, Ir, Rh and Ni. The first metal layer 105 can be formed by a conventional deposition process including, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), physical vapor deposition (PVD), sputtering, chemical solution deposition and plating. The deposition step is followed by a thermal anneal which reflows the first metal layer 105 into the feature by capillary action. Suitable thermal anneal conditions are given below in association with the cobalt reflow. In preferred embodiments, the thickness of the first metal layer 105 will be sufficient to fill a first portion of the feature. In preferred embodiments of the invention, the thickness of the first metal layer 105 is in the range of 1 nm to 100 nm, with a thickness from 2 nm to 20 nm being more typical. Although not depicted, after the reflow process, there is usually a very small amount of continues Cu on both sidewall and field (not shown in the figure).

By performing a first fill, the remaining aspect ratio of the feature for the next metal fill is reduced. A reduced aspect ratio is preferred for the second metal fill from both a process capability and deposited material quality point of view. Thus, embodiments of the invention use a two metal fill process rather than a single, complete fill of the feature in a single step.

FIG. 4 is a cross-sectional diagram depicting the structure after a Ru metal deposition step has been performed according to a first embodiment of the invention. The ruthenium layer 107 can be formed by a conventional deposition process including, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), atomic layer deposition (ALD), physical vapor deposition (PVD), sputtering, chemical solution deposition and plating. In preferred embodiments, the thickness of the Ru layer will be sufficient to cover the liner layer 103 and the first metal layer 105 and in the range of 1 nm to 100 nm, with a thickness from 2 nm to 20 nm being more typical. As illustrated, the Ru deposition layer 105 is substantially conformal over the rest of the structure, however, a conformal layer is not a requirement of the invention.

FIG. 5 is a cross-sectional diagram depicting the structure after a cobalt (Co) deposition step has been performed according to a first embodiment of the invention. The cobalt layer 109 is formed by a physical vapor deposition (PVD) in the preferred embodiment. Deposition of cobalt using other deposition techniques, for example, enhanced atomic layer deposition (ALD), chemical vapor deposition (CVD), sputtering, chemical solution deposition and plating have much greater impurities. For example, empirical data indicates that CVD and ALD depositions have C, Cl, O and S impurities cumulatively in excess of 1000 pm while PVD layers have cumulative deposition less than 200 ppm. Test data has indicated that purity of the Co layer 109 is important for creating a low resistivity conductor. Further, the purity of the cobalt layer should improve the reliability of the interconnect, and therefore, the device. In preferred embodiments, the thickness of the Co layer 107 is sufficient to fill a first portion of the feature after a subsequent thermal anneal step. In preferred embodiments, the thickness of the Co layer 109 will be in the range of 2 nm to 800 nm, with a thickness from 5 nm to 100 nm being more typical. Although as illustrated, the Co deposition layer 109 appears substantially conformal over the Ru layer 107, a PVD deposited film is generally not conformal nor is this a requirement of the invention. The presence of the Ru layer 107 improves the reflow properties of the Co in aggressively scaled features (less than 20 nm) and is expected to improve reliability of the interconnect. Having Ru/Co deposited and passivated on Cu provides an improved electromigration resistance because of a reduced Cu surface migration.

FIG. 6 is a cross-sectional diagram depicting the structure after a thermal anneal step has been performed according to a first embodiment of the invention according to an embodiment of the invention. As shown, the Co layer 109 fills a second, remaining portion of the feature. The desired depth of the second portion is dependent on the resistivity and reliability needed for the final interconnect structure.

In one preferred embodiment, the thermal anneal is carried out in a furnace between a temperature range between 200-500 degrees Centigrade in a neutral ambient, for example, in an N2, H2, He ambient or a mixture thereof. If carried out in a furnace, the thermal anneal is carried out for a period of 30 minutes to 5 hours in embodiments of the invention. In another embodiment, the thermal anneal is carried out through laser annealing. 20 nanoseconds to 30 minutes, 400-900 degrees Centigrade using a similar ambient.

Depending on the chosen temperature the anneal will form a Ru/Co interface or a Ru(Co) alloy liner. At relatively low temperatures, e.g., 300 degrees Centigrade and below, a Ru/Co interface is formed, at high annealing temperatures, a Ru(Co) alloy is formed. Experimental data indicates that when a Ru(Co) alloy is formed, its thickness is less than 5 nm. For a certain thermal platform, a higher annealing temperature requires a shorter time to complete the reflow process. The thermal anneal also reflows the cobalt 107 from the field area into the patterned features in the dielectric due to capillary driving force.

FIG. 7 is a cross-sectional diagram depicting the structure after a planarization step has been performed according to a first embodiment of the invention. The drawing depicts the structure after a planarization process such as a chemical mechanical polishing (CMP) step has been performed according to a first embodiment of the invention. Typically, a CMP process uses an abrasive and corrosive chemical slurry (commonly a colloid) in conjunction with a polishing pad. The pad and wafer are pressed together by a dynamic polishing head and held in place by a plastic retaining ring. As shown, the CMP step has removed the excess portions of the liner layer 103, the Ru layer 107 and the Co layer 109 in the field areas of the dielectric layer outside the features of the pattern in the dielectric 101. As mentioned above, in embodiments of the invention where a high anneal temperature is used and an Ru(Co) alloy layer formed, the Ru(Co) alloy layer is also removed by the CMP step. Other planarization processes are known to the art and are used in alternative embodiments of the invention.

FIG. 8 is a cross-sectional diagram depicting the structure after a planarization step has been performed according to a second embodiment of the invention. This figure corresponds to FIG. 7 which depicts the structure for the first embodiment after a chemical mechanical polishing (CMP) step or other planarization has been performed. In FIG. 8, a nitridized layer 111 is formed before the liner layer 103 of FIG. 2. In the drawing, a surface treatment has been performed on the dielectric substrate resulting in a nitridized surface layer 111. The nitridized layer 111 is created on the surface of the dielectric including the sidewalls and bottom of the feature utilizing a plasma or thermal process which increases the concentration of nitrogen in a surface portion of the dielectric. Using both layers 111 and 103 in a structure for extra adhesion between the first metal layer 105 and the dielectric 101 may be critical for semiconductor products which require high reliability.

The thermal nitridation process employed in embodiments of the present invention disclosure does not include an electrical bias higher than 200 W in a nitrogen-containing gas or gas mixture. The nitrogen-containing gases that can be employed in the present invention include, but are not limited to, N2, NH3, NH4, NO, and NHx wherein x is between 0 and 1 or mixtures thereof. In some embodiments, the nitrogen-containing gas is used neat, i.e., non-diluted. In other embodiments, the nitrogen-containing gas can be diluted with an inert gas such as, for example, He, Ne, Ar and mixtures thereof. In some embodiments, H2 can be used to dilute the nitrogen-containing gas. The nitrogen-containing gas employed in the present disclosure is typically from 10% to 100%, with a nitrogen content within the nitrogen-containing gas from 50% to 80% being more typical. In one embodiment, the thermal nitridation process employed in the present disclosure is performed at a temperature from 50° C. to 450° C. In another embodiment, the thermal nitridation process employed in the present disclosure is performed at a temperature from 100° C. to 300° C. for 30 minutes to 5 hours. In one set of embodiments, the resulting nitride enhanced layer is between 2 angstroms to 30 angstroms thick, but alternative embodiments can have thicknesses outside this range.

In some embodiments, a N2 plasma process is used to create the nitride layer which involves an electrical bias higher than 350W. An N2 plasma can be controlled without damaging the dielectric with ion current density range: 50˜2000 uA/cm2, and process temperature between 80 and 350 degrees C.

FIG. 9 is a cross-sectional diagram depicting the structure after a planarization step has been performed according to a fourth embodiment of the invention. The processing is similar to that in the first embodiment, except that an additional nitridation step has been added after the Ru deposition so that at least a nitridized surface is created on the Ru layer 107′ (denoted as 107′ rather than 107 to indicate the change). The nitridized Ru layer 107′ is created utilizing a plasma or thermal process which increases the concentration of nitrogen in at least a surface portion of the Ru layer 107′. In some embodiments, only a surface layer of ruthenium is converted into a nitrogen enriched layer. In the embodiment as depicted, the Ru layer 107′ is fully nitridized. The conditions used in the nitridation step can be similar to those discussed above in relation to nitridation of the dielectric surface. The presence of the Ru nitride layer 107′ improves the reflow properties of the Co in aggressively scaled features (less than 20 nm) and is expected to improve reliability of the interconnect. In addition, the nitridized layer prevents the Ru and Co from forming an alloy. For a certain thermal platform, a higher annealing temperature requires a shorter time to complete the reflow process, so the nitridized layer is an advantage when a separation of the Ru and Co layers should be maintained, but a higher annealing temperature is desired.

FIG. 10 is a cross-sectional diagram depicting the structure after a planarization step has been performed according to a fourth embodiment of the invention. The processing is similar to that in the second embodiment, having both the nitrogen enriched layer 111 and the liner layer 103 to promote adhesion. In addition, the nitridation step of the third embodiment has been added after the Ru deposition so that a nitridized surface is created on the Ru layer 107′ or a fully nitridized Ru layer 107′ is created.

Processing of additional layers of the integrated circuit device proceeds after the steps illustrated in the disclosure. For example, a second set of conductive lines could be created using an embodiment of the invention in subsequent steps if required for completion of the integrated circuit.

The resulting structure can be included within integrated circuit chips, which can be distributed by the fabricator in wafer form (that is, as a single wafer that has multiple chips), as a bare die, or in a packaged form. In any case, the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.

While only one or a limited number of features are illustrated in the drawings, those ordinarily skilled in the art would understand that many different types of features could be simultaneously formed with the embodiment herein and the drawings are intended to show simultaneous formation of multiple different types of features. However, the drawings have been simplified to only show a limited number of features for clarity and to allow the reader to more easily recognize the different features illustrated. This is not intended to limit the invention because, as would be understood by those ordinarily skilled in the art, the invention is applicable to structures that include many of each type of feature shown in the drawings.

While the above describes a particular order of operations performed by certain embodiments of the invention, it should be understood that such order is exemplary, as alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, or the like. References in the specification to a given embodiment indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic.

In addition, terms such as “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “upper”, “lower”, “under”, “below”, “underlying”, “over”, “overlying”, “parallel”, “perpendicular”, etc., used herein are understood to be relative locations as they are oriented and illustrated in the drawings (unless otherwise indicated). Terms such as “touching”, “on”, “in direct contact”, “abutting”, “directly adjacent to”, etc., mean that at least one element physically contacts another element (without other elements separating the described elements).

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 

Having described our invention, what we now claim is as follows:
 1. An integrated circuit device comprising: a substrate including a dielectric layer patterned with a pattern includes a set of features in the dielectric layer for a set of metal conductor structures; an adhesion promoting layer disposed on the set of features in the patterned dielectric layer; a metal layer filling a first portion of the set of features disposed on the adhesion promoting layer; a ruthenium layer disposed on the metal layer; and a cobalt layer disposed on the ruthenium layer filling a second portion of the set of features, the second portion a remainder portion of the set of features not filled by the adhesion promoting layer, the metal layer and the ruthenium layer, wherein the cobalt layer is completely formed using a physical vapor deposition process.
 2. The device as recited in claim 1, wherein the adhesion promoting layer is a liner layer.
 3. The device as recited in claim 1, wherein the adhesion promoting layer is a nitrogen enriched layer formed in the patterned dielectric layer produced by a nitridation process.
 4. The device as recited in claim 1, wherein the adhesion promoting layer is comprised of a nitrogen enriched layer formed in the patterned dielectric layer produced by a nitridation process and a liner layer comprised of one or more materials selected from the group consisting of Ta, Ti, W, TaN, TiN and WN.
 5. The device as recited in claim 1, wherein the set of metal conductor structures are a set of conductive lines.
 6. The device as recited in claim 1, wherein a top surface of the cobalt layer in the set of metal conductor structures is coplanar with a top surface of the dielectric in field areas of the dielectric layer.
 7. The device as recited in claim 1, wherein the set of metal conductor structures are a set of conductive vias.
 8. The device as recited in claim 1, wherein the set of metal conductor structures are a set of conductive lines and a set of vias.
 9. The device as recited in claim 1, further comprising a RuCo alloy layer disposed between the ruthenium layer and the cobalt layer.
 10. The device as recited in claim 1, wherein the set of features have a first dimension of less than twenty nanometers.
 11. The device as recited in claim 1, wherein first metal layer has a thickness in the range of 2 nm to 20 nm and the thickness of the Co layer has a thickness in the range of 2 5 nm to 100 nm.
 12. The device as recited in claim 1 further comprising a nitrogen enriched surface layer in the ruthenium layer. 